Various constructions and techniques will be described below. However, nothing described herein should be construed as an admission of prior art. To the contrary, Applicants expressly preserve the right to demonstrate, where appropriate, that anything described herein does not qualify as prior art under the applicable statutory provisions.
Cemented Carbide inserts and articles have been commercially available for use as cutting tools, wear parts and dies for many years. Typical cemented carbides are comprised of metal carbides, normally WC, often with the addition of carbides of other metals such as Ti, Ta, Nb, V, Zr, etc., and a metallic binder comprising Co, Ni, Fe or combination thereof. Various combinations of binders and metal carbides are mixed together in a body to produce the desired characteristics of hardness, toughness, and chemical and abrasion resistance. Cemented WC parts incorporating a binder in nominal concentrations between about 2 and 30 weight %, and cubic carbides such as TiC, TaC, NbC, VC and ZrC, and combinations thereof in concentrations up to about 30% by weight of the total weight have the requisite characteristics for most applications useful to the automobile and other industries. The parts formed from such cemented carbides are often coated with one or more refractory layers to impart desired characteristics which may be lacking in the substrate material or to otherwise improve performance of the finished article. Known coatings include Al2O3, ZrO2, Y2O3, AlN, cBN, as well as nitrides and carbonitrides of Groups IVA and VA, and combinations thereof.
Parts combining various amounts of metal carbides and binders, as well as different carbide phases, have been developed in attempts to optimize performance. U.S. Pat. Nos. 4,743,515 and 5,856,626 from Fischer et al. attempted to improve the strength of cobalt cemented carbide by creating a two-layered body utilizing eta-phase. Eta-phase is understood in the industry to mean compositions of tungsten, cobalt and carbon, such as M6C and M12C, where M=tungsten and cobalt, for example W3CoC. However it is known in the art that eta-phase forms brittle grains around WC crystals, providing sites for crack initiation and propagation. The presence of eta-phase results in a marked reduction in strength of the resulting article. Fischer et al. disclose parts having an inner layer comprised of WC, Co and eta-phase, with an outer layer which was eta-phase-free. In the substrates described by Fischer et al., the cobalt concentration in the eta-phase-free outer layer varies with depth from about 10-90% of the nominal value at the surface, to at least 120% of nominal, and then drops sharply in the inner eta-phase containing layer. The method for achieving the two layers requires sintering powders having substochiometric quantities of carbon at high temperature to generate eta-phase and then transforming the outer layer of eta-phase via high temperature carburization.
U.S. Pat. No. 5,453,241 by Akerman et al. seeks to improve the toughness of products produced per Fischer et al. by establishing a method of high temperature carburization followed by rapid cooling. All of the foregoing patents share the drawback of containing eta-phase, which is brittle and acts as a source for fracture initiation and propagation. Furthermore, the use of temperatures greater than 1400° C. for post sintering heat treatments has the drawback of loss of geometric features, warpage, and a reduction in hardness, due to excessive grain growth. Another drawback of this prior art is the presence of porosities in the substrate which are detrimental to the performance of the finished article.
Modifications can also be carried out by increasing the concentration of the binder phase in the near surface regions of the part. This binder phase enrichment improves certain properties of the part, such as toughness, but has the drawback of leaving residual binder at the surface, which interferes with later coating of the part. U.S. Pat. Nos. 5,560,839; 5,660,881; 5,618,625; and 5,713,133 detail the removal of the binder from the surface by etching, grinding, and other means followed by coating of the article with diamond. A drawback to these methods is increased porosity in the body and damage to the WC grains at the surface. U.S. Pat. No. 5,380,408 by Svensson details a method of removing cobalt from the surface of a part having a cobalt enriched surface region by chemical etching without removal of cobalt channels between the hard material grains. Removal of only the surface cobalt is asserted to improve coating adherence without creating undesirable porosity in the part.
Another drawback of the cobalt enriched surface region in the cemented carbide is a resulting decrease in hardness and chemical wear resistance, particularly when machining super alloys, such as titanium and its alloys. This lack of hardness leads to accelerated tool wear, even when coatings are applied to the part. Alternative materials such as eta-phase containing cemented carbides, discussed above, and selected cemented carbides consisting of WC with very small amounts of cobalt and ceramics, are thought to lack sufficient toughness to withstand forces associated with repeated use of tools, wear parts and dies. It is therefore desirable to produce a cemented carbide part having a combination of hardness and toughness, which overcomes the drawbacks of the prior art.